2025/12/22
In MOSFET switching, the gate driver plays a critical role in charging and discharging the gate, and the underlying energy conversion process directly affects the efficiency and thermal design of the driver system. Although the conventional power loss equations are widely used, they can lead to misunderstandings in certain applications. This article re-examines the actual energy flow in driver circuits by analyzing several typical charging/discharging models, and further discusses the impact of parasitic inductance on system energy conservation, providing engineers with a more accurate basis for energy estimation and device selection.
The driver charges the MOSFET as shown in Figure 1, and discharges it as shown in Figure 2:
Figure 1: Charging
Figure 2: Discharging
The power loss is calculated using the following equations:
Where:
QG is total gate charge at the end of charging.
fDRV is gate drive frequency.
VDRV is drive voltage.
QG*fDRV represents the average charging current.
VDRV*QG*fDRV represents the average power supplied by the source.
Equations (1) and (2) divide this power equally: half dissipated in the resistors and half stored in the capacitor. During discharge, the energy stored in the capacitor is dissipated through the resistor.
Clearly, the condition for equations (1) and (2) to hold is that the energy dissipated in the resistor equals the energy stored in the capacitor during charging. However, is this assumption always valid? Obviously not when the resistance is zero. What about when resistance is non-zero?
The MOSFET charging waveform is shown in Figure 3, and the I-V curve in Figure 4.
Stage (1): The MOSFET is in the cutoff region, where the capacitance is CGATE= CGS+CGD.
Stage (2): The MOSFET is in the saturation region, where the capacitance is CGATE=CGS+CGD*(1+gm*RLOAD).
Stage (3): The MOSFET is in the saturation region, where the capacitance is CGATE=CGD*(1+ gm*RLOAD).
Stage (4): The MOSFET is in the linear region, where the capacitance is CGATE=CGS+CGD.
CGS and CGD can be found in the MOSFET datasheet, where CISS= CGS+CGD, CRSS= CGD.
Due to the Miller effect in the saturation region, CGD is amplified by a factor of (1+AV/V), where AV/V represents the amplification factor of the MOSFET in the saturation region.
CGD varies with voltage. For most MOSFETs, the following approximation formula applies:
At stages (1), (2), and (4), CISS can be approximated as CGS in parallel with CGD_AVG. At Stage (3), VGS remains nearly constant, rendering CGS ineffective, and the driver charges CGD with a constant current.
(1) Power loss during constant-current charging in stage (3) of Figure 3
Energy dissipated in resistor
Energy stored in capacitor
When 
, , the energy dissipated in the resistor equals the energy stored in the capacitor. When , the resistor dissipates more energy than the capacitor stores.Energy supplied by the source
In real-world circuits, when a driver IC charges the MOSFET, most of the charging current is supplied by a capacitor, allowing the driver circuit to be approximated as a model of capacitor-to-capacitor charging.
(1) Power loss during constant-current charging in stage (3) of Figure 3
Energy dissipated in resistor
Energy stored in capacitor
Energy supplied by the source capacitor
Compared to the formula for constant voltage source charging capacitor, is replaced by , because the source capacitor voltage decreases during charging, so the average value over the charging process is used.
(2) Power Loss in RC Charging (Stages 1, 2, and 4 of Figure 3 Combined)
Let the initial voltage of the source capacitor be , its instantaneous voltage be; gate capacitance ; voltage of at the end of charging ; charging duration is T; charging current is . As shown in Figure 5, voltage and current are solved using the s-domain model:
